We have developed a model for studying tolerance to persistent low dose antigen in vivo, which results in the generation of a large number of anergic (hyporesponsive) T cells. We call this state adaptive tolerance. We inject CD4+, cytochrome c-specific T cells from a T cell receptor transgenic mouse on a Rag2-/- background (a monospecific T cell population) into a second transgenic mouse (called mPCC) expressing the cytochrome c antigen under the control of the MHC class I promoter and an immunoglobin heavy chain enhancer. Within 24 hours after transfer, the T cells are all activated by the antigen (as evidenced by an increase in size and expression of CD69), and proliferate extensively for several days, increasing in number about 100-fold. This expansion is followed by a deletional phase during which 50% of the cells disappear. Finally, the population reaches a steady state level in which the cells appear to be refractory to restimulation in vivo and in vitro. In this adaptive tolerant state, cytokine responses to high doses of antigen in vitro are inhibited 90%. In vivo BrdU labeling shows a slow T cell turnover of about 5% per day. This hyporesponsive state is reversible if the cells are transferred again into a second host not expressing the antigen. Interestingly, if the retransfer is into a host expressing the antigen, the cells remain hyporesponsive and slowly decrease their IL-2 and IFN gamma production by another 6-10 fold over 3-4 weeks. This deeper state of anergy suggests that the tolerance process is adaptable to different levels. [unreadable] During the past year we have studied the impact of this T cell intrinsic antigen adaptation on peripheral immune tolerance. Overlapping roles have been ascribed for T cell anergy, clonal deletion and regulation in the maintenance of peripheral immunological tolerance. However, a measurement of the individual and additive impacts of each of these processes on systemic tolerance is generally lacking. Using our model system we have attempted to tease out the unique contribution of T cell intrinsic receptor calibration (adaptation) in the maintenance of tolerance to a systemic self antigen. In our experiments we demonstrate that the CD4 T cell expansion to the persistent self antigen results in a polyclonal antibody response by the resident B cells. The antibodies are directed against a variety of self antigens classically implicated in autoimmune disorders, such as anti-DNA antibodies. The mice eventually develop a mild form of chronic arthritis. Interestingly, CD4+ T cells that were first stably adapted to the antigen in the absence of B cells, still retained the ability to help B cells and to cause autoimmune disease. In contrast, in the presence of an endogenous repertoire of T cells, the ability of the autoreactive T cells to induce pathology was severely compromised. This tolerance was correlated with a failure of the transferred transgeneic T cells to complete effector differentiation and survive in the T-cell replete host. Overall, our data demonstrate that at least two distinct mechanisms, one cell intrinsic and the other in trans, must cooperate to ensure complete peripheral tolerance.